Studies are proposed whose goal is to develop a molecular level understanding of the visual receptor protein, rhodopsin, and to relate this to the functions that rhodopsin performs as a photoreceptor. In order to do this, it is proposed to: (1) obtain improved two-dimensional crystals of frog rhodopsin that will allow a higher resolution projection map to be achieved; (2) obtain three-dimensional crystals of rhodopsin from enzymatically-cleaved bovine rhodopsin, chemically-modified rhodopsin, from a complex of rhodopsin with an Fab or Fv antibody, and from rhodopsin from several different species; (3) express and study the properties of rhodopsin from a mesophilic and a thermophilic species of ant in hopes of identifying a thermostable species of rhodopsin suitable for crystallization; (4) investigate new promising approaches to rhodopsin crystallization using quasisolid lipidic cubic phases, "peptitergents" and peptide additives; (5) test hypotheses for how rhodopsin is responsible for "dark noise" by determining the thermal stability of rhodopsin from species that differ in dark noise, and by determining retinal binding affinity; (6) determine the amino acid sequence in rhodopsin's carboxyl terminal region that is responsible for rhodopsin's vectorial transport; and (7) develop and maintain data bases for vertebrate and invertebrate rhodopsin that will aid in determining phylogenetic relationships and structure-function relationships.